This was the first thing that came into my mind when i tried my Vive Pro for the first time. The Spring is now available on thingiverse: https://www.thingiverse.com/thing:2896509
The only design flaw with the headphones is the missing pressure that a normal headphone strap would apply against your ears to make the speakers fit tight against your ears. This way most of the air inside is sealed and almost all frequencies can build up the sound pressure they need. Without some sort of spring, the speakers are just to loose and sound pressure (important for lower frequencies) is lost.
Another important thing, when i started my Vive Pro for the first time, i also had the bad audio quality trouble, but then i realized that the Vive Pro is about 4 USB devices in one package (Tracking Sensors, Blutooth Communication, Dual Webcam, Audio Interface) so make sure you connect it to a free powerful USB 3/3.1 controller that doesn't share it's bandwidth with other devices. Now everything sounds fine, i use HDMI audio and the volume buttons are working.